In such a TDMA system (see, for example, commonly owned U.S. Pat. No. 4,215,348) the messages to be sent out arrive concurrently over local channels at a concentrating terminal associated with the transmitting ground station for sampling, digital coding and arrangement in time slots temporarily allocated to respective channels in a recurrent outgoing PCM frame, normally of 125 .mu.s. Such a frame may have n=32 time slots, two of them (e.g. the 1.sup.st and the 17.sup.th) being reserved for synchronizing and supervisory signals including address codes identifying the destinations of the messages conveyed in the remaining 30 time slots. The digitized message sample in each active time slot is normally an 8-bit byte, corresponding to a sampling rate of 64 kbit/sec. The overall bit rate of such a PCM frame is 2 Mbit/sec.
The characteristics of TDMA terminals present at the ground stations designed for satellite links are internationally specified by the most important space authorities, such as Intelsat and Eutelsat.
It is well known that at a TDMA ground station a 2 Mbit/sec PCM stream is to undergo various operations before being utilized to modulate the phase of the radio-frequency carrier used for transmission to the satellite. More particularly, sixteen 8-bit samples or bytes of each channel can be grouped together in orderly sequence. Within a given time interval of 2 ms, which may be referred to as a TDMA frame period, the 128 bits of such a of p=16 bytes sequence are converted to a higher speed and combined with the bits of other PCM streams by a multiplexing operation, thus yielding a standard outgoing TDMA frame of the same duration as the initial sequence.
On the other hand, precise specifications relating to SS/TDMA installations have not yet been internationally defined. In particular, the time corresponding to the shortest switching unit or packet aboard the satellite is still to be standardized.
A value corresponding to thirty DSI (Digital Speech Interpolation) channels, namely sixty telephone circuits, is an acceptable digital-sequence length. This value affords a reasonable frame efficiency, given as the ratio between the length of the utilized frame portion to the overall frame length, and is compatible with the aforementioned 2 Mbit/sec PCM transmission systems carrying 32 channels of 64 kbit/sec, i.e. 30 channels for speech, one channel for signaling and one channel for synchronization.
This technique, however, is unsatisfactory in areas of low traffic density where only a few messages at a time are intended for a given remote ground station.